The dynamic response of a rear suspension assembly of a tracked vehicle such as a snowmobile, to the multitude of loads imposed upon it during operation, has a significant affect on the overall performance and rider comfort of the vehicle. Three types of loads are regularly exerted upon a tracked vehicle. The impact loads imposed upon the rear suspension as the vehicle travels rough terrain and encounters bumps are of the most concern. There are also internal forces developed during rapid acceleration, which causes a weight transfer from the front of the vehicle to the rear. This tends to lift the skis off the ground and thus interferes with steering. Furthermore, there are centrifugal forces imposed on the vehicle when the vehicle makes turns at high speeds. The complex interaction of the forces which occur in the rear suspension assembly during vigorous operation have demanded optimal design of mechanisms for absorbing and attenuating the complex combination of loads imposed upon a modern high performance snowmobile.
The fundamental structure of the rear suspension of a tracked vehicle such as a snowmobile has remained essentially constant for many years. Conventionally, the rear suspension supports the endless track, which is tensioned to surround a pair of parallel slide rails, a plurality of idler wheels and at least one drive wheel or sprocket. A shock absorbing mechanism involving compressed springs, hydraulic dampers, etc., urges the slide frame assembly and the chassis (also known as a frame) of the snowmobile apart, against the weight supported above the suspension in a static condition.
One example of a conventional rear suspension of a snowmobile is described in U.S. Pat. No. 5,727,643, issued to Kawano et al. on Mar. 17, 1998. Kawano et al. discloses a suspension device for providing a resilient support for a snowmobile body, including a frame for supporting the snowmobile body. A slide rail is operatively connected to the frame for pressing a crawler belt against a snow surface. A swing arm includes a first end pivotally supported on the frame and a second end pivotally mounted on the slide rail. A shock absorber assembly includes a first end pivotally supported on a shaft adjacent to the first end of the swing arm, a second end of the shock absorber assembly being connected to the frame through a progressive link pivotally supported on the swing arm.
Another example of a conventional rear suspension of a snowmobile is disclosed in U.S. Pat. No. 5,904,216, issued to Furusawa on May 18, 1999. Furusawa discloses a rear suspension of a snowmobile including two angular suspension arm assemblies, which connect the slide frame assembly to the snowmobile chassis. These suspension arm assemblies are moveable independently of one another in order to permit the slide frame assembly to accommodate itself to static and dynamic forces arising during operation. A single cushion unit extends horizontally and is operatively connected at opposed ends thereof to the respective suspension arm assemblies in order to support and attenuate the loads.
Irregularities in the terrain traveled by the snowmobile produce displacements and deflections of the front suspension that supports the vehicle on the skis, and of the rear suspension. Depending upon their magnitude, frequency and strength, these deflections cause more or less discomfort to the operator and passenger of the snowmobile. It has been recognized that suspension displacements that produce angular acceleration of the snowmobile and its operator about a transverse horizontal axis, produce more discomfort than displacements that merely produce vertical acceleration of the vehicle and its operator. Therefore, coupled suspension systems have been developed, in which the pivotal movement of the two suspension arm assemblies are coupled and the slide rails are thereby generally kept parallel to the vehicle so that the operator and passenger of the snowmobile are less affected by the uneven terrain.
Although conventional rear suspension systems available provide a relatively comfortable ride to the passengers, it is desirable to further improve the rear suspension assemblies for tracked vehicles, particularly snowmobiles.